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Internet Engineering Task Force (IETF)                          D. BiderRequest for Comments: 6668                               Bitvise LimitedUpdates:4253                                                 M. BaushkeCategory: Standards Track                         Juniper Networks, Inc.ISSN: 2070-1721                                                July 2012SHA-2 Data Integrity Verification forthe Secure Shell (SSH) Transport Layer ProtocolAbstract   This memo defines algorithm names and parameters for use in some of   the SHA-2 family of secure hash algorithms for data integrity   verification in the Secure Shell (SSH) protocol.  It also updatesRFC4253 by specifying a new RECOMMENDED data integrity algorithm.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6668.Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Bider & Baushke              Standards Track                    [Page 1]

RFC 6668              Sha2-Transport Layer Protocol            July 20121.  Overview and Rationale   The Secure Shell (SSH) [RFC4251] is a very common protocol for secure   remote login on the Internet.  Currently, SSH defines data integrity   verification using SHA-1 and MD5 algorithms [RFC4253].  Due to recent   security concerns with these two algorithms ([RFC6194] and [RFC6151],   respectively), implementors and users request support for data   integrity verification using some of the SHA-2 family of secure hash   algorithms.1.1.  Requirements Terminology   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].2.  Data Integrity Algorithms   This memo adopts the style and conventions of [RFC4253] in specifying   how the use of new data integrity algorithms are indicated in SSH.   The following new data integrity algorithms are defined:   hmac-sha2-256     RECOMMENDED   HMAC-SHA2-256                                   (digest length = 32 bytes,                                    key length    = 32 bytes)   hmac-sha2-512     OPTIONAL      HMAC-SHA2-512                                   (digest length = 64 bytes,                                    key length    = 64 bytes)                          Figure 1   The Hashed Message Authentication Code (HMAC) mechanism was   originally defined in [RFC2104] and has been updated in [RFC6151].   The SHA-2 family of secure hash algorithms is defined in   [FIPS-180-3].   Sample code for the SHA-based HMAC algorithms are available in   [RFC6234].  The variants, HMAC-SHA2-224 and HMAC-SHA2-384 algorithms,   were considered but not added to this list as they have the same   computational requirements of HMAC-SHA2-256 and HMAC-SHA2-512,   respectively, and do not seem to be much used in practice.Bider & Baushke              Standards Track                    [Page 2]

RFC 6668              Sha2-Transport Layer Protocol            July 2012   Test vectors for use of HMAC with SHA-2 are provided in [RFC4231].   Users, implementors, and administrators may choose to put these new   MACs into the proposal ahead of the REQUIRED hmac-sha1 algorithm   defined in [RFC4253] so that they are negotiated first.3.  IANA Considerations   This document augments the MAC Algorithm Names in [RFC4253] and   [RFC4250].   IANA has updated the "Secure Shell (SSH) Protocol Parameters"   registry with the following entries:   MAC Algorithm Name      Reference       Note   hmac-sha2-256RFC 6668        Section 2   hmac-sha2-512RFC 6668        Section 2                        Figure 24.  Security Considerations   The security considerations ofRFC 4253 [RFC4253] apply to this   document.   The National Institute of Standards and Technology (NIST)   publications: NIST Special Publication (SP) 800-107 [800-107] and   NIST SP 800-131A [800-131A] suggest that HMAC-SHA1 and HMAC-SHA2-256   have a security strength of 128 bits and 256 bits, respectively,   which are considered acceptable key lengths.   Many users seem to be interested in the perceived safety of using the   SHA2-based algorithms for hashing.5.  References5.1.  Normative References   [FIPS-180-3]              National Institute of Standards and Technology (NIST),              United States of America, "Secure Hash Standard (SHS)",              FIPS PUB 180-3, October 2008, <http://csrc.nist.gov/publications/fips/fips180-3/fips180-3_final.pdf>.   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-              Hashing for Message Authentication",RFC 2104, February              1997.Bider & Baushke              Standards Track                    [Page 3]

RFC 6668              Sha2-Transport Layer Protocol            July 2012   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC4231]  Nystrom, M., "Identifiers and Test Vectors for HMAC-              SHA-224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",RFC 4231, December 2005.   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)              Transport Layer Protocol",RFC 4253, January 2006.5.2.  Informative References   [800-107]  National Institute of Standards and Technology (NIST),              "Recommendation for Applications Using Approved Hash              Algorithms", NIST Special Publication 800-107, February              2009, <http://csrc.nist.gov/publications/nistpubs/800-107/NIST-SP-800-107.pdf>.   [800-131A] National Institute of Standards and Technology (NIST),              "Transitions: Recommendation for the Transitioning of the              Use of Cryptographic Algorithms and Key Lengths", DRAFT              NIST Special Publication 800-131A, January 2011,              <http://csrc.nist.gov/publications/nistpubs/800-131A/sp800-131A.pdf>.   [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)              Protocol Assigned Numbers",RFC 4250, January 2006.   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)              Protocol Architecture",RFC 4251, January 2006.   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",RFC 6151, March 2011.   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security              Considerations for the SHA-0 and SHA-1 Message-Digest              Algorithms",RFC 6194, March 2011.   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms              (SHA and SHA-based HMAC and HKDF)",RFC 6234, May 2011.Bider & Baushke              Standards Track                    [Page 4]

RFC 6668              Sha2-Transport Layer Protocol            July 2012Authors' Addresses   Denis Bider   Bitvise Limited   Suites 41/42, Victoria House   26 Main Street   GI   Phone: +1 869 762 1410   EMail: ietf-ssh2@denisbider.com   URI:http://www.bitvise.com/   Mark D. Baushke   Juniper Networks, Inc.   1194 N Mathilda Av   Sunnyvale, CA 94089-1206   US   Phone: +1 408 745 2952   EMail: mdb@juniper.net   URI:http://www.juniper.net/Bider & Baushke              Standards Track                    [Page 5]